Excited-state hydrogen bonding effect on dynamic fluorescence of coumarin 102 chromophore in solution: A time-resolved fluorescence and theoretical study

The steady-state absorption and emission as well as the time-resolved fluorescence spectra of coumarin 102 (C102) in both aprotic and alcoholic solvents have been used to study the effect of excited-state hydrogen bond on the dynamic fluorescence of C102 chromophore in various solutions. The dual fluorescence of C102 in alcohols, which is dependent on the hydrogen-bonded donation ability of the solvent, has been assigned to the distribution of free C102 and a hydrogen-bonded complex. Furthermore, a shift of the fluorescence spectra induced by excited-state hydrogen bond has been demonstrated to take place within hundreds of picoseconds by the performance of the time-resolved fluorescence spectra with the time-correlated single-photon-counting (TCSPC) technique. Moreover, the time-dependent density functional theory (TDDFT) has been used to calculate the hydrogen-bonded equilibrium constant pKHB in different electronic states. It has been demonstrated for the first time that the hydrogen bond strengthening in electronic excited states could decrease the free energy of the hydrogen-bonded complex due to its stronger binding energy. Therefore, the hydrogen-bonded equilibrium will become markedly in favor of the hydrogen-bonded forms in electronic excited states by comparison with the case in the ground state.